Relaxation oscillator



l.. w. HERSHINGER 2,476,090

RELAXATION OSCILLATOR 2 shets-sneet 1 July 12, 1949.

Filed March 9, 1948 @1mm-M July 12, 1949 x w. HERSHINGER 2,476,090

RELAXATION oscILLAToR 4 Filed March 9, 1948 2 Sheets-Sheet 2 /r/G. 2d.

F76. Z C.

m6,@ @mi I JN VEN TOR. www 14./ #Nif/W65@ Patented July 12, 1949 RELAXATION OSCILLATOR Lincoln W. Hershinger, Oreland, Pa., assigner to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application March 9, 1948, Serial No. 13,931

(Cl. Z50- 36) l Claims.

The invention herein described and claimed relates to a relaxation oscillator of improved frequency stability. More particularly, the present invention provides an improved blocking tube oscillator whose free-running frequency is inherently very stable, being substantially unn affected, for example, by variations in plate voltage. Such variations in plate voltage may be caused either by an unregulated plate voltage supply or may be caused by adjustments made manually to the D.C. plate voltage to control, for example, the amplitude of the oscillators output voltage.

Due to the inherent stability of my improved circuit When operating as a free-running blocking tube oscillator, I am able to lock-in the oscillator, in frequency and in phase, with regularly recurring trigger pulses of substantially smaller amplitude than have heretofore been required to eect dependable synchronization. Consequently, in a preferred circuit arrangement, suitable means are included for limiting to a small value the amplitude of the synchronizing pulses. The amplitude of noise pulses are then also limited to a small value. This is very advantageous in that the period of time during which the improved circuit is responsive to noise pulses, when operating as a synchronized blocking tube oscillator, is reduced to an extremely small interval relative to that obtaining in prior art synchronized oscillators. This will become clear as the description proceeds.

It is a broad object of this invention to provide a relaxation oscillator yof improved frequency stability.

It is another object of this invention to pro-y vide a synchronized blocking tube oscillator Whose frequency is stabilized against variations in plate voltage and against electrical noise disturbances.

It is a more specific object of this invention to provide a blocking tube oscillator capable of being synchronized in a dependable manner with regularly-recurring trigger pulses of substantially smaller amplitude than have heretofore been required to insure satisfactory synchronization or lock-in.

Another object of this invention is to provide a blocking tube oscillator of excellent frequency stability adapted for use in the deflection circuits, particularly the horizontal deilection circuits, of a television receiver.

These and other objects, advantages, and features of the present invention, and the manner in which the objects are attained, will become l clear from a consideration of the following detailed description and accompanying drawing in which:

Figure l is a schematic representation of a preferred form of my improved blocking tube oscillator shown in the environment of the deflection system of a television receiver, and

Figures 2a-2d are graphical representations of various voltage waveforms which will be helpful in explaining and understanding the present invention.

Referring now to Figure 1, a blocking tube oscillator embodying the present invention is Shown to comprise a rst triode Ill and a second triode 2t. Plate ll of first triode lil is coupled,

by Way of capacitor 26, to cathode 23 of second triode 2t, cathode 23 being connected to ground by Way of a cathode load resistor 24. Coupling capacitor 26 and cathode load resistor 2li together form a network 46 whose RC time constant is one of the factors which determines the free-running frequency of the improved blocking tube oscillator, as will be described.

Plate 2l of second triode 2U is connected, by Way of the primary Winding 29 of transformer 2l and resistor 34, to a source of positive plate voltage, B+. A bypass capacitor 3|' is connected between the plate end of resistor 3ll and ground. Where the output of the improved blocking tube oscillator is intended to be a sawtooth voltage, as is ordinarily the case Where the oscillator is employed in the deflection system of a television receiver, capacitor 3| is the element across which the sawtooth output voltage is developed in conventional manner. In Figure l, the sawtooth output voltage is shown to be applied, by Way of coupling capacitor 33, to output utilization means 35 The plate-cathode output circuit of second triode 2t is inductively coupled, by means of transformer 2l', to the grid-cathode circuit of first triode I0. In the circuit shown in Figure l, a source i8 of synchronizing pulses is also coupled to the grid-cathode circuit of tube lil by way of an amplitude limiter circuit l 9, a series capacitor Il, a shunt load resistor I3, and the secondary Winding 28 of transformer 21. The employment of an amplitude limiter circuit I9 is preferable but not essential. Load resistor I8 is preferably of small value in order that the synchronizing and noise pulse voltages developed thereacross may be of small magnitude. Cathode resistor i4 and capacitor I 5 comprise a cathode biasing net- Work 25 for rst triode Ill. It is to be understood that, when my improved circuit is employed as a free-running blocking tube oscillator, the connection of source I6 to grid I2 is not made, in Which case the low potential end of secondary winding 28 is connected directly to ground.

In accordance with the present invention, the

plate-cathode output circuit of first triode Ill` includes, in addition to the branch previously described (corhprising coupling capacitor 26 and` cathode loadtres'istor 243", another' branch which includes a load resistor 4I, and an LC network 40 comprising an inductance 42 in shunt with ai capacitance 43. Capacitance 45 is a la-rg'e'bypass capacitor. The low potential ends of inductance 42 and capacitance 43 are consequently at ground potential with respect to the opertirg'frduency' of the oscillator. The W potential end of inductance 42 is connected, by Way' of a voltagee. dropping resistor 44, toa source of plate supply voltage, B+.

The operation of rny improved `blocking tube oscillator will now' be' described'. Itwill faciliiate` the description' tov assume: that the' circuit is in oscillation.

pulsesL to either tube Hl or tube 2 0;

The description will begin' atvr th'a't, point the cycle of operation` at whichv tube 2U r'es. When tube 213 goes into conduction, plate current ows through primary winding 29 of transformer' 21' andthe potentialy at the pla'te end of Winding 29 drops sharply. A pulse of positivel voltage is' thereupon induced; by' Well known transformer' action, at the grid end of secondary winding28. This" positive pulse appears; ofcourse, at grid I2 of'tube IIJ. The constants of the'cathode biasing network 25 may, if desired', besuch' as to'permit some plate current to iioW in' tube I 'IlL prior to' the tiine the said pulse arrivesA atx grid |21 When vthe pulse appears o n' grid I2', the plate current of tube l0 increases sharply', the potential ofv plate" Il drops sharplyt and the potential of cathode 2.3

tri'ode' 2f! reaches its maximum value; This plate current will not remain constant atv its'` maxi-..1

mum value becausev the cathode and plate; po'- tentials of tube vwillichange due' to the'cliar'g-` ing of capacitor 2li4 by' thecathode current'irof tube 20- and the" failure of primary' inductance 29 to sustaina constantvoltage acrossits ter.' Ininals; Consequently, after reaching its'. maxi.- l

60 start tol decrease. Thisdecreasel results'Y in a' deg- 1 mum value, the platecurrent of' tube* 20 will crease in the positivev voltage applied to grid I2", an increase in the voltage atplate H", anj increase in voltage at cathode 23; andi a'ji'urth'er d'ecrea'sej inthe plate current of, tubej20; This action' is, cumulative, as before, and c'o'ntir1fu'esl sharplyuritil tube 20-ceases-to conduct; In'g'enferalpractice',y

It will further facilitate the r'st" portion7 of the description' toass'uxrle that the-circuit'is operating asa fr ee-running'lblockingf tube* oscillator Without application or' synchronizing` the conduction periods1 oftube's |50 and ZUareshort compared to the norl-conductionperiods.

N eglecting for the nioment'the effectjofvLC' net;`

work 40, it will be seen thatsec'ondktubejjwill' remain in a nonconductive state' tl'repfosif tive `charge which developedonl capacitor' 261d`urfing' coml'uctiony leaks off suilfcien tly to lower the,`

potential ofcathode v23 below-the cutoff` point' (equivalent to raising the grid potential above th cutoff point), whereupon tube 20 will again conduct. The cycle of operation described above is then repeated. The action thus far described is believed to be suiiciently similar to that which takes place in a conventional single-tube blocking oscillator to make descriptionV in greater detail unnec'es'sany'.y

The important effect of'LC network 40 and of resistor 4I upon the operation of the circuit will now be considered. The present invention proposesfthatl. as a preferred matter, inductance 42 and capacitance 43, which form LC network 40, be so' c'lio's'emV that network 40 will be resonant at ay fundamental frequency which is slightly lower than the frequency at which the blocking tube oscillator isides'ire'df to operate as a free-running device. If desired, the constants may be so chosen thatv network is resonant to a multiple of the said fundamental frequency.

The pulse of current which flows', as previously described, into'plat'e' llj of tube fup'asse's through the resonant circuit 40. Resonantc'i'rcit 4U' then Shook excited By the pulse. anda sineiw'ave voltage is thus develop'ed across" the' rr'etw lf.A

When network 4U is tuned to. a" frequency slightly lower than thefreei-runn'ingj operating' frequency".

of the blocking oscillator; as" is' preferred; LC network 40" is' shock' excited once dur'ng't'he n'e'g'a'- tive-going portion' of e'acli' cy`cl"e just" prior tof'tl'i'e' occurrence or the negative peak of` the" Sine' wave;

The Waveform' of tl'ievoltagedevelope'd aoross" a pulse of plate current. from' 'rst tube: ru 'newsinto capacitor 4'3 toreplacetl'ieenergylost du'rirg'f the sine-wave oscillation; Tlie voltage atV point" a thereupon' drop's rather sharply; thel'nagi't'u'fd of the' drop', relativer to the amplitude of.' tire' sine wave; being aj function, ofthel Q-fact'or' ofjresoat circuit 4 0. In Fig 2a; the, sharp" drop in the potential of point a i'sindicjatedby the steeplslope" ju'st, prior to the occurrenceof the negati'veipeak'sgi form 2a, developed' across' LCnletwork Minicom-'fbination with thenegative pi'ul's'eof voltagedevelencJ nrst tube m' conducts strongly.

The waveform. shown in Figure 2li-is applied, by Way' of capacitor 26",. to cathode` 2.3.. of.- secondi` tube; 20?. There, waveform. 2b oombinesiwitnaa.

exponential sawltootli vol'tagecomponent 2c. The

sawtooth waveform '2c'A represents the voltage,"

whichis present,acrosstcapacitor 12e. due to its being charged; rapidly.' byV the pulse of cathode. current in tube'll.'addschargedslowly ata rate determined by HLRC. tiile constant. ofl networkA llt.y Theresultafint voltage Waveformeatcathoce. 23' (point dii'n" Eigure',1).isshown,in i'gurefz'dl.

It' will be seen that waveform 2d' comprisestte.

combination of the sinefwavef-with-superimposedpulse' waveform 2 54 and tlrie` sawtooth :wavefform` 2c'.

My invention" contemplatesthe xcircuit.pa-ranceeters' of the' improved. blocking tube oscillator., to be sol selectedthat, Whenffthe oscillator is op erating', tubey 2li willfnotre until `thepotentalT of" cathode. 23 drops' .to a'- selected. value, which,. isy

only attained for' a very snort period during' each cycle. This is clearly depicted in Figure 2d where the cutoff potential of cathode 23 is indicated by the line so marked. Observe that the waveform 2d is so shaped that only for a short period during each cycle is the cathode potential sufficiently low (equivalent to the grid potential being sufficiently high) to permit second tube 2t to conduct. This short period begins when two of the three voltage components of which waveform 2d is comprised are moving in a negative-going direction. The third component, i. e. the negative pulse component, is not developed, in a freerunning blocking oscillator, until second tube 2i) res.

Stated another way, the circuit is so arranged and the parameters are so chosen that the ordinarily positive potential of cathode 23 is driven down sufficiently close to ground potential to permit the flow of plate current in tube 2li when the negative-going portion of the sine-wave voltage 2a combines with a low potential portion of the sawtooth voltage 2c to produce a resultant cathode potential of preselected low magnitude, represented in Figure 2d by the line marked cutoff. When the cathode potential reaches this value, second tube 3i) fires, and a negative pulse of voltage is developed at point b which, being applied to point d, assists and intensifies the conduction taking place in tube 2li.. Maximum platecurrent flow is quickly reached, and soon thereafter the plate current flow decreases in the manner previously described. The potentials of plate III of tube Iii and of cathode 23 of tube 2d thereupon rise rapidly, and second tube 2S cuts on. Thereafter, the potential of cathode 23 follows a path determined by the sine wave voltage component 2a in combination with the sawtooth voltage component 72e until a value is reached sufficiently close to ground to cause tube to again conduct. The frequency of the oscillation is consequently a function of both the RC time constant oi network l5 and the resonant frequency of LC network 4Q.

I have observed that the new circuit is capable of operating as a free-running blocking tube oscillator of excellent frequency Stability. Its freerunning frequency is, for example, substantially independent of variations which may occur in plate voltage. And, to an important extent, the free-running frequency is less affected by variations occurring in certain of the circuit constants which in prior art oscillators have considerable effect on the operating frequency.

The stability of my free-running blocking tube oscillator against variations in plate voltage will rst be discussed. As indicated hereinabove, plate voltage variations ,may be due to an unregulated supply or may be caused by adjustments made manually to the D.C. plate voltage to control, for example, the amplitude of the oscillators output voltage. Assume that the plate supply voltage increases, When this occurs, the plate currents of both tubes increase. The increase in the plate current of second tube 2S produces an increase in the amplitude of the exponential sawtootbvoltage component developed by RC network llt. The increase in the plate current of nrst tube la produces an increase in the amplitude of the sine wave voltage developed across LC network eid, and also in the amplitude of the negative pulse voltage developed across resistor 4I. Observe, hoy ever, that an increase in the amplitude of the positive-potential sawtooth voltage component tends to delay the time at which tube 2l) fires, Whereas an increase in the amplitude of the sine 6, wave voltage component tends' to advance the' time at which tube 20 res.

Stated another way, when the plate currents of both tubes increase as a result, for example, of an increase in the D.C. plate voltages, forces are developed which are in opposition to each other, one tending to increase the frequency of operation of the blocking tube oscillator, the other tending to decrease the frequency of operation. The circuit parameters may consequently be so selected that the opposing forces are of substantially equal magnitude, in which case no appreciable change in oscillator frequency will occur when the D.-C. plate voltage is varied. In my experiments, I have found that the free-running operating frequency remained substantially constant over a two-to-one change in plate voltage.

I indicated above that my improved blocking tube oscillator is less affected by variations which may occur in the values of certain of the frequency-determining circuit constants. I have found, for example, that small variations in the values of resistor 24 and/or capacitor 26 have a smaller effect upon the operating frequency than such variations would have had in prior art blocking oscillators. Assume, for example, that the value of resistor 2d changes during warm-up of the apparatus. In this event, the amplitude and time constant of the sawtooth voltage component 2c will change. As a consequence, the free-running frequency of the oscillator will tend to change, but the actual change will be relatively7 small. This stability is derived from the LC circuit Observe that the slope of the sine Wave voltage component 2a (which is substantially unaffected by the change in the constants of RC network lit) is substantially steeper, at the point corresponding to the time of firing of tube 20, than the slope of the sawtooth voltage component 2c. Consequently, the firing potential of cathode 23 is reached at very nearly the same time that it would have been reached had the constants of RC network et not changed slightly. Of course, changes of relatively large magnitude in the constants of RC network 46 will change the freerunning frequency of my blocking tube oscillator. As a matter of fact, cathode resistor 2Q may, in practice, be a variable resistor for controlling, in a Vernier-like manner, the free-running frequency of the oscillator.

The operation of my improved circuit as a synchronized, rather than as a free-running, blocking tube oscillator will now be described. When the frequency of my improved blocking tube oscillator is to be controlled by regularlyrecurring synchronizing or trigger pulses from an outside source, the circuit constants are so selected that, in the absence of the synchronizing pulses, the free-running frequency of the oscillator is slightly lower than the repetition irequency of the synchronizing pulses.

Assume that, in Figure 1, synchronizing pulses of positive polarity from source I6 are applied, by way of amplitude limiter I9, coupling capacitor Il', shunt load resistor I8, and secondary winding 28, to grid I2 of rst triode II) at a time when the circuit is operating as a free-running oscillator. In practice, amplitude limiter I9 may comrise the synchronizing-separator or pick-off circuit of the television receiver, which, by virtue of plate saturation limits the pulses to a uniform amplitude. When a positive synchronizing pulse arrives at grid I2, the plate current of tube Ill rises sharply and a pulse of negative-going voltage is developed at point b which is applied, by Way 9 duration of one cycle of operation, which is 63.5 microseconds. At the instant tube 20 fires, ca pacitor 23 has therefore discharged to an appreciable extent, and the voltage thereacross is con Sequently changing at a relatively slow rate. The free-running operating frequency of the oscillator is therefore determined largely by the resonant frequency of the sine wave voltage de veloped across network di).

The synchronizing pulse voltages (and also the noise pulse voltages) applied to grid' i2 are developed across load resistor Iii, which is shown in Figure 1 to be of small Value. Hence, the an plitude of the pulse voltages is small.

Having described a preferred embodiment of my invention, I claim:

1. A blocking tube oscillator comprising: a first Vacuum tube having at least plate, cathode and grid electrodes; a second vacuum tube having at least plate and cathode electrodes; an external path interconnecting the plate and cathode electrodes of said second tube, said path including an inductance and a cathode load resistance; means for connecting a source of direct-current voltage to the plate of said second tube; a first external path interconnecting the plate and cathode electrodes of said first tube, said path including a sine wave-forming parallel-resonant inductance-capacitance network; a second external path interconnecting the plate and cathode electrodes of said first tube, said second path including a capacitance and said cathode load resistance of said second tube, said second path being in shunt with at least a portion of said first path; means for connecting a source of direct-current voltage to the plate of said first tube; and an external path interconnecting the grid and cathode electrodes of said rst tube, said path including an inductance mutually coupled to said first-mentioned inductance.

2. A blocking tube oscillator comprising: a first Vacuum tube having at least plate, cathode and grid electrodes; a second vacuum tube having at least plate and cathode electrodes; an external path interconnecting the plate and cathode electrodes of said second tube, said path including an inductance and a cathode load resistance; means for connecting a source of direct-current voltage to the plate of said second tube; a first external path interconnecting the plate and cathode electrodes of said first tube, said path including a sine wave-forming parallel-resonant inductance-capacitance network tuned to a frequency slightly lower than n times the frequency at which the oscillator is desired to operate, 11. being an integer; a second external path interconnecting the plate and cathode electrodes of said first tube, said second path including a capacitance and said cathode load resistance of said second tube, said second path being in shunt with at least a portion of said first path; means for connecting a source of direct-current voltage to the plate of said first tube; and an external path interconnecting the grid and cathode electrodes of said first tube, said path including an inductance mutually coupled to said first-mentioned inductance.

3. A blocking tube oscillator as claimed in claim 2 characterized in that said first plate-to-cathode circuit of said first tube includes a pulse-forming resistor.

4. A blocking tube oscillator as claimed in claim 2 characterized in that the factor n is one.

5. A blocking tube oscillator as claimed in claim 2 characterized in that the factor 1r. is an integer greater than one.

6. A blocking tube oscillator comprising: first and second Vacuum tubes having at least plate, cathode and grid electrodes; an external path interconnecting the plate and cathode electrodes of said second tube, said path including an inductn ance and a cathode load resistance; means for connecting a source of direct-current voltage to the plate of said second tube; a first external path interconnecting the plate and cathode electrodes of said first tube, said path including a sine waveforming parallel resonant inductance capacitance network and a pulse-forming load resistsance, said inductance-capacitance network being resonant to a frequency slightly lower than 1i times the frequency at which the oscillator is de sired to operate, n being an integer; a second external path interconnecting the plate and cathode electrodes of said first tube, said second path including a capacitance and said cathode load re sistance of said second tube, said second path being in shunt with at least a portion of said rst path, said capacitance and cathode load resistance functioning as an RC biasing network for said second tube, the RC time constant of said network being shorter than the time duration of one cycle of oscillation at the desired operating frequency; means for connecting a source of direct-current voltage to the plate of said rst tube; and an external path interconnecting the cathode and grid electrodes of said first tube, said path including an inductance mutually coupled to said first-mentioned inductance.

7. A blocking tube oscillator as claimed in claim 6 characterized in that the factor n is one.

8. A blocking tube oscillator as claimed in claim 6, characterized in that the factor n is an integer greater than one.

9. A blocking tube oscillator as claimed in claim 6 characterized in that means are provided for applying to a grid-cathode circuit of said oscillator regularly recurring voltage pulses whose repetition frequency is substantially equal to the frequency at which said oscillator is desired to operate, whereby said oscillator is synchronized in frequency and in phase with said regularly recurring pulses.

10. A blocking tube oscillator as claimed in claim 1, characterized in that said second vacuum tube has also a grid electrode and in that said grid electrode is connected effectively to the lowpotential end of said cathode load resistance of said second tube.

LINCOLN W. HERSHINGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,125,732 Bowman-Manifold et al Aug. 2, 1938 2,182,555 Geiger Dec. 5, 1939 2,277,000 Bingley Mar. 17, 1942 

